The encoder is widely utilized for a positional detection of a robot arm or else, and is constructed to read a scale of the encoder plate attached to a rotation shaft of a motor, by means of a detection element. The positional detection encoder includes an incremental type and an absolute type. The former type is constructed such as to read a position of the rotary disc by counting an incremental pulse with reference to an origin of the rotary disc. The latter type undergoes the positional detection by reading a code formed on the rotary disc regardless of a mechanical state thereof. Therefore, the incremental type needs to rotate at most one cycle to restore the origin when restarting the encoder operation after a power source has been shut down, whereas the absolute type advantageously does not need the origin restoring operation because the position can be instantly read without moving the rotary disc when the power source is recovered.
FIG. 13 shows a typical structure of the conventional absolute encoder. A rotary disc 101 is formed thereon with a plurality of concentric tracks 102-105. Each track is comprised of an annular slit pattern which is bit-coded according to a digital code system indicative of an absolute position of the rotary disc 101. A photodetector array 106 is opposed to one side of the rotary disc 101 , and a. photoemitter such as an LED 108 is opposed to the other side of the disc through a stationary slit plate 107. The slit pattern formed in the rotary disc 101 selectively passes and cuts off a light beam from the LED 108 so that the photodetector array 106 outputs detection signals according to a light intensity variation on respective tracks of the array 106. These detection signals are processed so as to read the angular absolute position or angular address of the rotary disc 101. Namely, this address is represented by the aforementioned digital code.
There have been known various digital code systems for representing an address. FIG. 14 schematically shows a slit pattern formed according to a regular binary code which is one example of the digital code systems. The illustrated pattern diagram indicates track numbers in a left column and addresses in a top row. The slit pattern of each track is binarily coded, and is composed of bright and dark sections. In this example, there are provided four tracks corresponding to four bits so as to represent 2.sup.4 =16 number of absolute addresses. Such a regular binary code is a basic arrangement in the digital process. However, at a transition from one address to another address, switching between adjacent bright and dark sections may occur concurrently at two or more tracks. It is quite difficult to just concurrently detect respective transitions, thereby causing the drawback that a reading error may be generated by transitional fluctuation of detection timings.
FIG. 15 shows Gray code which is designed to remove the above noted drawback. As seen from the pattern diagram, the Gray code is characterized in that the switching between adjacent bright and dark sections occurs only on one track at every transition of the addresses in contrast to the regular binary code, thereby effectively avoiding the reading error. However, the Gray code requires a multiple of tracks corresponding to a number of bits likewise the regular binary code. Therefore, as the bit number is increased to multiply addresses for achieving higher resolution power, a multiple of tracks are arranged in parallel manner along a radial direction of the rotary disc to thereby hinder down-sizing of the absolute encoder.
FIG. 16 shows a slit pattern formed according to a binary coded quaternary system which can effectively reduce a number of tracks by half. A pair of tracks "0" and "1" are assigned with two binary bits for representing a quaternary lower order. For example, addresses "0-"3" belonging to a first group are discriminated from each other by the quaternary numbers of the lower order. Similarly, addresses "4"-"7" belonging to a second group are also discriminated from each other by the quaternary numbers of the lower order. The same is true for the remaining third and fourth groups. Another pair of tracks "2" and "3" are assigned with two binary bits for representing a higher order of the quaternary number. The first, second, third and fourth groups are discriminated from each other by the higher order of the quaternary numbers. This binary coded quaternary system can be readily converted into the regular binary system by a simple logical computation. For example, in the FIG. 16 arrangement, provided that a bit signal P.sub.0 is obtained from the track "0", a bit signal 191 is obtained from the track "1", a bit signal 192 is obtained from the track "2", and a bit signal P.sub.3 is obtained from the track "3", the first bit or the lowest order bit B.sub.0 of the regular binary code is produced by exclusive logical OR operation of P.sub.0 and P.sub.1. The second bit B.sub.1 is identical to P.sub.0. The third bit B.sub.2 is produced by exclusive logical OR operation of P.sub.2 and P.sub.3. The fourth or highest order bit B.sub.3 is identical to P.sub.2.
FIG. 17 shows two different arrangements of a photodetector array for reading the binary coded quaternary slit pattern. A left photodetector array 110 has four photodetectors arranged in line correspondingly to the respective tracks "0"-"3". On the other hand, a right photodetector array 111 has a pair of photodetectors arranged along the track "1" by a phase difference of 90.degree. with each other relative to the periodic slit pattern of the track "1" This pair of photodetectors can read a quaternary number of the lower order. Similarly, another pair of photodetectors are arranged along the track "3" by a phase difference of 90.degree. with each other so as to read a quaternary number of the higher order. By such an arrangement, unnecessary tracks "0" and "2" can be eliminated to thereby reduce the number of tracks by half. To facilitate better understanding to this point, supplementary description is given again with reference to FIG. 16. The pair of tracks "0" and "1" have periodic slit patterns having the same period but a relative phase difference of 90.degree.. Accordingly as illustrated by FIG. 17, the first pair of photodetectors are shifted from each other by the phase difference of 90.degree. to enable reading of all the information contained in the tracks "0" and "1". Namely, the first pair of photodetectors can receive four combinations of bright and bright, bright and dark, dark and bright, and dark and dark to read the quaternary information. In similar manner, the tracks "2" and "3" shown in FIG. 16 have periodic slit patterns having the same period but a relative phase difference of 90.degree.. Therefore, the arrangement of the photodetector array 111 of FIG. 17 can read quaternary information of the higher order. Further, a plurality of photodetector arrays 111 may be arranged periodically along the periodic slit patterns to ensure a sufficient intensity of the received light. For the simplicity, the above description is directed to the relative relation between the photodetector array and the tracks in the FIG. 17 diagram. Practically, a stationary mask plate may be utilized to define desired photodetecting areas in place of separated photodetectors. As described above, the binary coded quaternary slit pattern can effectively reduce the number of tracks by half. However, switching between the bright and dark sections may occur concurrently on two or more of the parallel tracks in contrast to the previous Gray code pattern. Therefore, the binary coded quaternary pattern still suffers from the transitional reading error.
As opposed to the above noted code patterns which need a plurality of parallel tracks, another type of the absolute encoder utilizes a single track, as disclosed for example in Japanese Patent Application Laid-Open No. 3-6423. As shown in FIG. 18, a rotary disc has a single absolute track formed with bright and dark slits arranged at different pitches, and patterned according to M-sequence code. A photodetector array is provided circumferentially to read an absolute address of the rotating disc. However, the M-sequence slit pattern does not have a periodic structure, hence a multiple of photodetector arrays cannot be disposed in the circumferential direction, thereby failing to raise the received light amount. Consequently, it would be difficult to achieve high resolution because a sufficient light amount is not obtained.
In view of the above noted drawbacks of the prior art, a first object of the invention is to reduce a number of tracks for down-sizing and higher resolution, while maintaining a sufficient amount of received light by multiple photodetection along a periodical slit pattern and removing a reading error. Particularly, the present invention is directed to removal of the reading error.
Referring back to FIG. 13, brief description is given to another problem of the prior art to be solved. For the simplicity, the four number of tracks are formed on the rotary disc 101 in the exemplified FIG. 13 prior art. Each track is recorded with a one-bit data so that the rotary disc 101 is written with absolute addresses composed of four-bit parallel data. Only sixteen absolute addresses are designated by the four-bit parallel data. Practically, a rotary disc is recorded with 12-bit or 16-bit parallel data to obtain high resolution power. In such a case, the lowest order track has a quite fine slit pattern pitch which is 1/2.sup.12 or 1/2.sup.16 as short as that of the highest order track. However, an actual minimum pitch is limited over a certain dimension. If the slit pattern pitch is reduced to the order of micron meter, incident light beam may be diffracted to seriously degrade S/N ratio. Accordingly, the FIG. 13 optical system utilizing the linearity of the light has a lower limit of the minimum slit pattern pitch. In a higher resolution mode, slit pattern pitches of respective tracks are set proportionally to the minimum pattern pitch of the lowest order track, so that the slit pattern pitch of the highest order track is made significantly long. A diameter of the rotary disc must be enlarged in order to contain such a long pitch slit pattern circumferentially of the disc, thereby hindering scale-down of the absolute encoder. Further, a certain radial space must be provided between adjacent tracks in order to avoid interference of incident illuminating lights between the tracks. The higher the resolution power, the greater the number of tracks, resulting in more need for a space adversely which enlarges the diameter of the rotary disc, thereby hindering scale-down of the absolute encoder.
In view of the above noted drawbacks of the prior art, a second object of the invention is to improve an optical system of the absolute encoder to realize micronization of the slit pattern to thereby achieve higher resolution power and further scale-down of the absolute encoder.